FIG. 1 shows an exemplary multi-hop wireless network. Referring to FIG. 1, a multi-hop wireless network comprises a number of wireless stations, which can be classified as a base station (BS), a relay station (RS), and subscriber station (SS) according to how the station functions in terms of delivery of packets from/to outer networks. A subscriber station (SS), such as SS 101 of FIG. 1, may or may not be mobile. When a subscriber station is mobile, it is sometimes denoted as a mobile station (MS).
In the downlink, a base station 102 introduces packets of outer networks into the multi-hop network, while subscriber station 101 consumes or terminates the packets. In the uplink, subscriber station originates packets while base station 102 introduces the packets into outer networks. A relay station, such as RS1, RS2 or RS3, resides on the path between base station 102 and subscriber station 101, and relays packets for either uplink or downlink. For purposes herein, a subscriber station may include, but is not limited to, phones, laptops, desktops, PDAs, and so on.
Through one or more switches and/or routers (not shown), base station 101 is connected to an outer network (not shown), which may be either a wired network or a wireless network. The outer network may be again a multi-hop wireless network.
In a multi-hop wireless network, a wireless link may serve as an access link, a relay link or both. With reference to FIG. 1, a wireless link serves as an access link if packets being transmitted over the link are received or transmitted by a subscriber station that is attached to the link. A wireless link serves as a relay link if packets being transmitted over the link are received and transmitted by a base station or a relay station. A wireless link serves both as a relay link and an access link if packets being transmitted over the link are received by both a relay station and a subscriber station.
The coverage of a base station and a relay station may take various shapes according to antennas, obstacles, channel conditions, and so on. Depending on its location and the deployment of base station and relay stations, a subscriber station may be within the coverage of one or more other wireless stations.
To increase the receiving performance using macro diversity, transmissions over multiple links are often synchronized. For example, to increase the receiving performance of a subscriber station, transmission at the multiple access links associated with the subscriber station can be synchronized. The transmission can be a unicast transmission to the specific station (as in soft handoff and cooperative communication) or a multicast or broadcast transmission to a plurality of stations (as in multicast and broadcast service). As another example, to increase the receiving performance of a relay station or a base station, transmission at the multiple relay links associated with the relay station or the base station can be synchronized. The transmission at the synchronized multiple-link may be either uplink (in the direction toward the base station) or downlink (in the direction toward the subscriber stations). Hereinafter, for purposes herein, the related art and embodiments of the present invention are illustrated with reference to the synchronized transmission in the access links that are downlink. Therefore, the synchronized links are assumed be an access link, although in general they can be access link, relay link and so on. Also, the packets for the synchronized transmission are assumed to be introduced by the base station, delivered in the downlink direction, and consumed by the subscriber stations, although in general the packets may be generated and consumed by any of the base station, relay stations, and subscriber stations, and the packet can transmitted either in downlink direction or uplink direction. It should be noted, however, the present invention can be easily extended to the general cases where such assumptions may not necessarily hold.
On receiving multiple signals from multiple transmitters (each transmitter being either base station or relay station), a subscriber station may apply diversity combining such as selection combining and maximal-ratio combining to obtain improved signal.
Synchronized transmission requires delay control along the paths to the access links in which the synchronized transmission is to occur. Because of the difference in the number of relay stations in the path, the difference in propagation delays in each link, the difference in the processing delay within each relay station, and the difference in channel status, the delay along a path may not necessarily the same with each other. The transmission time for synchronized transmission, therefore, is generally determined by taking into account the longest delay out of the delays in all the paths. For the packet transmission along the path other than the path incurring the longest delay, an artificial delay must be introduced so that transmission in multiple links should be synchronized.
Synchronized transmission methods for multicast and broadcast service are known. In one such method, each relay station reports its processing delay to the base station as a capability parameter. The base station then determines the maximum cumulative delay of all relay stations in the multicast and broadcast service zone based on their positions in the tree and their individual processing delays. The base station then calculates the required waiting time, Wi, for each relay station based on the value of the maximum cumulative delay and each relay station's cumulative delay and notify each relay station of its waiting time. When synchronized multicast and broadcast transmission is necessary, the base station forwards the data over the relay downlink as a pre-transmission a number of frames equal to the maximum cumulative delay before transmitting this data over the access link. Each relay station in the multicast and broadcast zone forwards the data it receives over the relay downlink. Finally, once the base station has waited a number of frames equal to the maximum cumulative delay and each relay station has waited its specified waiting time, Wi, the base station and relay station synchronously transmits the data over the access link. However, this particular approach has no considerations on the channel errors in the wireless link. If a channel error occurs for a packet transmitted in a link, all the relay stations and the access stations in the forward path from the link cannot receive the packet correctly; hence, the packet is not available for the synchronized transmission.
In another synchronized transmission method for multicast and broadcast service, each relay station reports its processing delay to the base station as a capability parameter and the base station then determines the maximum cumulative delay of all relay stations in the multicast and broadcast service zone based on their positions in the tree and their individual processing delays as in the previously described synchronized transmission method; in contrast, however, in this synchronized transmission method, the base station determines target transmission frame over access link for each synchronized data burst based on maximum cumulative delay and other base station information. If ARQ is enabled over a relay link, the base station may select target transmission frame to accommodate the delay due to ARQ retransmission on the relay link. The base station includes the frame number of the target transmission frame with each packet. The relay station transmits the packet to subscriber stations over access link at target transmission frame. The intermediate relay station relays the packet to its subordinate relay station based on the constraints of QoS parameters for the relay connection for the packet. During connection setup for a subscriber station, if a relay path with suitable characteristics, such as per-hop QoS configuration, is not available, the base station may initiate the creation of a new relay path with the required characteristics, such as per-hop QoS configuration. The QoS parameters of any of the constituent per-hop connections on an existing relay path may be changed by the base station.
Also, in this synchronized transmission method, if a relay station fails to transmit a packet at its target transmission frame, the relay station informs the base station of the failure. The relay station includes the duration of late arrival for this packet in units of frames. In addition, a relay station may provide early arrival information to the base station if the relay station determines a packet has arrived earlier than Relay Data Early Arrival Report Threshold to its target transmission frame. With early arrival detection, the relay station includes the number of frames exceeding the threshold that the packet has waited to be transmitted. When Relay Data Early Arrival Report Threshold is set to 0, the early arrival reporting is disabled.
FIG. 2 illustrates an exemplary ARQ state diagram for a transmitter. When a packet is transmitted, ARQ state transits to the “Outstanding” state. If ACK is received within ARQ_RETRY_TIMEOUT, ARQ state transits to the “Done” state. If ARQ_RETRY_TIMEOUT expires without receiving ACK or if NACK is received from the receiver, the packet is retransmitted while ARQ state is transiting from “Outstanding” to “Waiting for retransmission” and then from “Waiting for retransmission” to “Outstanding.” If ARQ_BLOCK_LIFETIME expires while ARQ state is in the “Outstanding” state or in the “Waiting for retransmission” state, the packet is discarded. Therefore, a packet may be either correctly transmitted or discarded after ARQ_BLOCK_LIFETIME, and the longest delay happens when it is discarded. ARQ_BLOCK_LIFETIME is chosen so that one or more retransmissions can be accommodated before the lifetime expires.